1. Field of the Invention
This invention relates to hydraulic jacks for use in elevating the load carriage and the mast structure of a forklift truck, and more particularly to an improved structural arrangement for mechanically and hydraulically interconnecting a cluster of hydraulic cylinders of the hydraulic jacks associated with the load carriage elevating and mast structure elevating functions of a forklift truck.
2. Description of the Prior Art
It has been known in the prior art of hydraulic controls for forklift trucks to provide a cluster of three hydraulic cylinders which are mechanically and hydraulically interconnected in such manner that hydraulic fluid admitted to the interior of a secondary cylinder passes through a hydraulic interconnection between the cylinders to a pair of contiguously disposed primary cylinders to cause movement of the forklift load carriage relative to the supporting mast structure; and whereby after the load carriage has reached the limit of its vertical movement on the portion of the mast structure upon which the load carriage is supported, further admission of hydraulic fluid to the secondary cylinder causes a vertical elevating movement of the portion of the mast structure on which the load carriage is supported to thereby further vertically elevate the load carriage relative to the chassis structure of the forklift truck. Clustered hydraulic cylinders which cooperate to assist in the vertical displacement of the forklift load carriage and of the mast structure of the forklift truck in the general manner just described are shown, for example, by U.S. Pat. Nos. 2,678,746 issued to C. D. Gibson on May 18, 1954, 2,821,264 issued to B. I. Ulinski on Jan. 28, 1958; and 3,489,249 issued to H. A. Stammen on Jan. 13, 1970. All three of the United States Patents just listed show two-section masts associated with a cluster of three hydraulic jacks, including a fixed or primary mast section which is fixed relative to the forklift vehicle chassis and a secondary mast structure which is telescopically movable relative to the primary or fixed mast structure. A forklift truck which is provided with a cluster of three mechanically and hydraulically interconnected hydraulic cylinders but in which the forklift truck is provided with a triple section mast including an outer mast section which is fixed against vertical movement relative to the vehicle chassis and also with an inner mast section and an intermediate mast section which are vertically movable relative to each other and also relative to the fixed outer mast section is shown in co-pending U.S. patent application Ser. No. 536,029 of Benonie C. Ehrhardt, entitled "Multiple Hose Guide Arrangement for a Lift Truck," filed Dec. 23, 1974, now U.S. Pat. No. 3,968,859, and assigned to the same assignee as the present application.
A disadvantage of the hydraulic cylinder cluster arrangement shown by the aforementioned prior art is that the plurality of cylinders constituting the cluster are rigidly mechanically connected together as by welding in such manner that the individual cylinders of the cluster cannot be separately removed for replacement or servicing, and if one damaged hydraulic cylinder of the cylinder cluster of the prior art requires replacement, it is necessary to replace the entire cluster assembly. Similarly, in the prior art if it was necessary to service one of the hydraulic cylinders it was necessary to remove the entire cluster of cylinders in order to service a single hydraulic cylinder.